When Loki had Sif's hair, Freyr's ship Skíðblaðnir, and Odin's spear Gungnir fashioned by the Sons of Ivaldi, he bet his own head with Brokkr that his brother Eitri (Sindri) would not have been able to make items to match the quality of those mentioned above.
So to make gifts to Freyr, Eitri threw a pig's skin into a furnace as Brokkr worked on the bellows, and together they manufactured the boar Gullinbursti which had bristles in its mane that glowed in the dark.
[Þ]á lagði Sindri svínskinn í aflinn ok bað blása Brokk ok létta eigi fyrr en hann tæki þat ór aflinum, er hann hafði í lagt. En þegar er hann var genginn ór smiðjunni, en hinn blés, þá settist fluga ein á hönd honum ok kroppaði, en hann blés sem áðr, þar til er smiðrinn tók ór aflinum, ok var þat göltr, ok var burstin ór gulli. ... Þá bar fram Brokkr sína gripi ... En Frey gaf hann göltinn ok sagði, at hann mátti renna loft ok lög nótt ok dag meira en hverr hestr ok aldri varð svá myrkt af nótt eða í myrkheimum, at eigi væri ærit ljós, þar er hann fór; svá lýsti af burstinni.
Sindri laid a pigskin in the hearth and bade Brokkr blow, and did not cease work until he took out of the hearth that which he had laid therein. But when he went out of the smithy, while the other dwarf was blowing, straightway a fly settled upon his hand and stung: yet he blew on as before, until the smith took the work out of the hearth; and it was a boar, with mane and bristles of gold. ... Then Brokkr brought forward his gifts: ... to Freyr he gave the boar, saying that it could run through air and water better than any horse, and it could never become so dark with night or gloom of the Murky Regions that there should not be sufficient light where he went, such was the glow from its mane and bristles.
The boar is also known as Slíðrugtanni (sometimes Anglicized to "Slidrugtanni").